1. Field of the Invention
This invention relates to a laser beam scanning apparatus to be used in a laser applied apparatus such as a laser beam printer, a laser COM (Computer Output Microfilming) system or the like.
2. Description of the Related Art
The laser beam printer or the laser COM system which uses laser beams emitted from a laser beam emitting source, records images by laser beams directly modulated on the basis of the image data.
Accordingly, it is necessary to stabilize the laser beam emission at all times so as to record images faithful to the image data. However, the emission from the laser beam emitting source is easily influenced by temperatures and the intensity changes owing to the deterioration of the emitting source itself. Moreover, even if the intensity itself does not change, the intensity on a beam receiving surface can be changed by any contamination of the optical system, thus making it difficult to obtain a proper image.
Conventionally, in order to deal with the change in the emission from the laser beam emitting source, the intensity of the laser beams is controlled to be constant on the basis of the output from a photosensor which detects the laser beams from the laser beam emitting source (Auto Power Control: APC).
Moreover, an apparatus is disclosed in U.S. Pat. No. 4,201,994, to correct a laser beam intensity on the basis of the output from a photosensor which detects the laser beam passed through the optical system in order to meet the lowering of the laser beam intensity at the beam receiving surface due to the contamination of the optical system.
In the case where the photosensor is provided to detect the laser beam which has passed through the scanning optical system, the intensity of the laser beams reaching the photosensor considerably decreases if the scanning optical system is excessively contaminated. In consequence, the output from the photosensor works as a signal to increase the laser beam intensity, which may cause an excessive supply of power to the laser beam emitting source and may eventually break the laser beam emitting source.